Conventionally, a method using partially saponified polyvinyl alcohol (hereinafter, polyvinyl alcohol may be abbreviated as PVA) as a dispersion stabilizer for suspension polymerization of a vinyl compound (such as vinyl chloride) has been known. However, the partially saponified PVA has low solubility in water, and it is difficult to increase the solid content concentration, resulting in insufficient handleability. For such reasons, in order to improve the handleability, attempts to use an aqueous emulsion as a dispersion stabilizer for suspension polymerization have been made. For example, Patent Documents 1 and 2 disclose methods using an aqueous emulsion as a dispersion stabilizer for suspension polymerization of a vinyl compound.
Further, in recent years, the dispersion stabilizer is required to have performance such as: (1) allowing a vinyl polymer that has high plasticizer absorption and that can be easily processed to be obtained even if the used amount is small; (2) having a small number of fish-eyes when the vinyl polymer is formed into a sheet even if the used amount is small; (3) reducing or suppressing hue deterioration of the vinyl polymer due to heat in the formation, even if the used amount is small; and (4) having excellent handleability of the dispersion stabilizer.
Patent Document 1 discloses a dispersion stabilizer for suspension polymerization of a vinyl compound composed of an aqueous emulsion with a high ratio of a graft polymer. The dispersion stabilizer can increase the solid content concentration to a concentration of about 15%, and the handleability is slightly improved. However, it is difficult to increase the solid content concentration any more, and the handleability and the economic efficiency are insufficient. Further, other performance is also insufficient. Patent Document 2 discloses a dispersion stabilizer for suspension polymerization composed of an aqueous emulsion with a high ratio of a graft polymer. In the dispersion stabilizer for suspension polymerization, the viscosity is reduced by reducing the amount of polyvinyl alcohol, so that the solid content concentration can be increased to about 30%. Therefore, the handleability is more improved than in the dispersion stabilizer disclosed in Patent Document 1. However, the handleability is still insufficient.